The present invention relates to contact lenses which are wearable, on a continuous basis, for extended periods of time. In particular, the invention relates to flexible, hydrophilic silicon-containing contact lenses which have advantageous combinations of properties.
Contact lenses are fundamentally classified into soft and hard type lenses. Hard contact lenses are literally hard and can be somewhat uncomfortable to wear. On the other hand, soft contact lenses are more comfortable to wear, but are commonly removed from the eye at the end of each day. Soft contact lenses are classified as hydrogel lenses and non-hydrogel lenses.
Conventional soft hydrogel contact lenses are often composed of copolymers of hydrophilic monomers, such as hydroxyethylmethacrylate, N-vinylpyrrolidone and the like, and can be prepared by lathe-cutting methods, spin casting methods, cast molding methods or combinations thereof, followed by a swelling treatment in a physiological saline and/or phosphate buffer solution to obtain lenses with water contents of about 20% or about 30% to about 80% by weight.
Soft silicon or silicone hydrogel contact lenses have been suggested for continuous wear for extended periods of time. For example, some silicone hydrogel contact lenses are intended to be worn overnight. Some silicone hydrogel contact lenses can be worn continuously for about two weeks, and some silicone hydrogel contact lenses can be worn continuously for about one month or about thirty days. Such continuous wear lenses have had relatively high oxygen permeabilities to provide for oxygen access to the cornea during the extended wearing of such lenses.
Oxygen permeability (Dk) is an important factor in contact lens design to maintain ocular health for contact lens wearers. As established by Holden and Mertz in 1984, a minimum of 87×10−9 (cm ml O2)/(sec ml mmHg) oxygen transmissibility is required for hydrogel contact lenses to limit overnight edema to 4% (Holden et al., Invest. Ophtalmol. Vis. Sci., 25:1161-1167 (1984)). Physical properties such as oxygen flux (j), oxygen permeability (Dk), and oxygen transmissibility (Dk/t) are used in referring to properties of contact lenses. Oxygen flux can be defined as a volume of oxygen passing through a specified area of a contact lens over a set amount of time. The physical units of oxygen flux can be described as μl O2 (cm2 sec). Oxygen permeability can be defined as the amount of oxygen passing through a contact lens material over a set amount of time and pressure difference. Physical units of oxygen permeability can be described as 1 Barrer or 10−11 (cm3 O2 cm)/(cm3 sec mmHg). Oxygen transmissibility can be defined as the amount of oxygen passing through a contact lens of specified thickness over a set amount of time and pressure difference. The physical units of oxygen transmissibility can be defined as 10−9 (cm ml O2)/(ml sec mmHg). Oxygen transmissibility relates to a lens type with a particular thickness. Oxygen permeability is a material specific property that can be calculated from lens oxygen transmissibility.
Oxygen transmissibility is commonly measured using polarographic and coulometric techniques known by persons or ordinary skill in the art. Oxygen permeability can be calculated by multiplying the oxygen transmissibility (Dk/t) of a lens by the mean thickness of the measured area. However, it appears that the polarographic techniques may not provide accurate measurements for high Dk silicone hydrogel contact lenses, such as silicone hydrogel contact lenses having a Dk greater than about 100 barrers. The variability associated with polarographic techniques may be related to the issue that for silicone hydrogel lenses having a Dk greater than 100 barrers, the measurements tend to plateau at Dk values greater than 100. The coulometric technique is frequently used to measure the Dk of lenses that are believed to have Dks greater than 100 barrers.
Prior art soft silicon-containing hydrophilic contact lenses with higher water contents tend to have reduced or lower oxygen permeabilities. For example, a silicone hydrogel contact lens available under the tradename, Focus Night & Day (available from CIBA Vision Corporation), has a water content of about 24% and a Dk of about 140 barrers. Another silicone hydrogel contact lens available under the tradename, O2 Optix (available from CIBA Vision Corporation), has a water content of about 33% and a Dk of about 110 barrers. Another silicone hydrogel contact lens available under the tradename, Acuvue Oasys (available from Johnson & Johnson), has a water content of about 38% and a Dk of about 105 barrers. Another silicone hydrogel contact lens available under the tradename, PureVision (available from Bausch & Lomb), has a water content of about 36% and a Dk of about 100 barrers. Another silicone hydrogel contact lens available under the tradename, Acuevue Advance (available from Johnson & Johnson), has a water content of about 46-47% and a Dk of about 65 barrers. In comparison, a non-silicone hydrogel contact lens available under the tradename, Acuvue2 (available from Johnson & Johnson), has a water content of about 58% and a Dk of about 25 barrers.
In addition, existing silicone hydrogel contact lenses have a modulus from between about 0.4 to about 1.4 MPa. For example, the Focus Night & Day contact lens has a modulus of about 1.4 MPa, the PureVision contact lens has a modulus of about 1.3 MPa, the O2 Optix has a modulus of about 1.0 MPa, the Advance contact lens has a modulus of about 0.4 MPa, and the Oasys contact lens has a modulus of about 0.7 MPa. In general, for existing silicone hydrogel contact lenses, as the Dk increases, the modulus of the lens increases.
Furthermore, existing silicone hydrogel contact lenses do not have desirable surface wettabilities. For example, the Focus Night and Day contact lens has a wetting angle of about 67°, the PureVision contact lens has a wetting angle of about 99°, the O2 Optix contact lens has a wetting angle of about 60°, and the Advance contact lens has a wetting angle of about 107°. In comparison, non-silicone hydrogel contact lenses have wetting angles of about 30°.
It is important that contact lenses be comfortable and safe to wear. For example, silicone hydrogel contact lenses should be comfortable and safe to wear for daily use, for overnight wear, and/or for wearing on an extended or continuous wear basis. One problem that arises in extended or continuous wear contact lenses is adhesion of the lens to the cornea during lens wearing which can result in wearer discomfort, eye irritation, corneal staining and/or other damage to the eye. Although lenses with high water contents are softer and more comfortable to wear, such prior art lenses may not have one or more properties useful to provide comfortable and safe wearing of the contact lenses. For example, existing contact lenses may not have a desirable Dk, a desirable surface wettability, a desirable modulus, a desired design, and/or a desirable water content. For example, silicone hydrogel contact lenses with a high Dk typically have a lower water content. In addition, such lenses are more stiff compared to lenses with a higher water content, and such lenses are less wettable.
To reduce stromal anoxia during daily wear of contact lenses, it is desirable to produce a lens that has an oxygen transmissibility of at least about 45. Lenses, such as certain existing silicone hydrogel contact lenses, with an oxygen transmissibility greater than 50 have been developed to reduce stromal anoxia during daily wear.
To help improve the properties of silicone hydrogel contact lenses, some lenses have been produced which include one or more surface treatments or surface modifications to attempt to make the lens surfaces more hydrophilic. Other lenses have been produced which include an interpenetrating network of polyvinylpyrollidone and a silicon-containing polymer.
There continues to be a need for new silicone hydrogel contact lenses which have advantageous combinations of properties such as, enhanced flexibility or less stiffness, better wettability, and/or better lens designs.